Keeping list of all sockets that were sent to child process causes memory
leak and thus unacceptable (see #4587). However `server.close()` should
still work properly.
This commit introduces two options:
* child.send(socket, { track: true }) - will send socket and track its status.
You should use it when you want `server.connections` to be a reliable
number, and receive `close` event on sent sockets.
* child.send(socket) - will send socket without tracking it status. This
performs much better, because of smaller number of RTT between master and
child.
With both of these options `server.close()` will wait for all sent
sockets to get closed.
Fedor Indutny